<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <link rel="shortcut icon" href="data:;base64,iVBORw0KGgo=">
  <style>
    html,
    body {
      margin: 0px;
      overflow: hidden;
      height: 100%;
    }
  </style>
</head>

<body>
  <script type="module">
    /* eslint-disable */
    // Contextual Typing
    {
      window.onmousedown = function (mouseEvent) {
        console.log(mouseEvent.button); //<- OK
        // console.log(mouseEvent.kangaroo); //<- Error!
      };
    }
  </script>
</body>

</html>